A study was conducted with the primary objective of examining the efficacy of a standard block shear test method to assess the bond quality of cross-laminated timber (CLT) products. The secondary objective was to examine the effect of pressure and adhesive type on the block shear properties of CLT panels. The wood material used for the CLT samples was Select grade nominal 25 x 152-mm (1 x 6-inch) Hem-Fir. Three adhesive types were evaluated under two test conditions: dry and vacuum-pressure-dry (VPD), the latter as described in CSA standard O112.10. Shear strength and wood failure were evaluated for each test condition.
Among the four properties evaluated (dry and VPD shear strength, and dry and VPD wood failure), only the VPD wood failure showed consistency in assessing the bond quality of the CLT panels in terms of the factors (pressure and adhesive type) evaluated. Adhesive type had a strong effect on VPD wood failure. The different performance levels of the three adhesives were useful in providing insights into how the VPD block shear wood failure test responds to significant changes in CLT manufacturing parameters. The pressure used in fabricating the CLT panels showed a strong effect on VPD wood failure as demonstrated for one of the adhesives. VPD wood failure decreased with decreasing pressure. Although dry shear wood failure was able to detect the effect of pressure, it failed to detect the effect of adhesive type on the bond quality of the CLT panels.
These results provide support as to the effectiveness of the VPD block shear wood failure test in assessing the bond quality of CLT panels. The VPD conditioning treatment was able to identify poor bondline manufacturing conditions by observed changes in the mode of failure, which is also considered an indication of wood-adhesive bond durability. These results corroborate those obtained from the delamination test conducted in a previous study (Casilla et al. 2011).
Along with the delamination test proposed in an earlier report, the VPD block shear wood failure can be used to assess the CLT bond quality. Although promising, more testing is needed to assess whether the VPD block shear wood failure can be used in lieu of the delamination test. The other properties studied (shear strength and dry wood failure), however, were not found to be useful in consistently assessing bond line manufacturing quality.
A study was conducted with the primary objective of gathering information for the development of a protocol for evaluating the surface quality of cross-laminated timber (CLT) products. The secondary objectives were to examine the effect of moisture content (MC) reduction on the development of surface checks and gaps, and find ways of minimizing the checking problems in CLT panels. The wood materials used for the CLT samples were rough-sawn Select grade Hem-Fir boards 25 x 152 mm (1 x 6 inches). Polyurethane was the adhesive used. The development of checks and gaps were evaluated after drying at two temperature levels at ambient relative humidity (RH).
The checks and gaps, as a result of drying to 6% to 10% MC from an initial MC of 13%, occurred randomly depending upon the characteristics of the wood and the manner in which the outer laminas were laid up in the panel. Suggestions are made for minimizing checking and gap problems in CLT panels. The checks and gaps close when the panels are exposed to higher humidity.
Guidelines were proposed for the development of a protocol for classifying CLT panels into appearance grades in terms of the severity of checks and gaps. The grades can be based on the estimated dimensions of the checks and gaps, their frequency, and the number of laminas in which they appear.
North American cross laminated timber is currently made of softwood lumber following the guidelines of the ANSI/APA PRG-320 manufacturing standard. In this study, the potential of manufacturing CLT panels using various hardwood species and engineered wood products (EWP) was investigated for their compatibility and the impact on the dimensional stability and aesthetics of the end products. Yellow birch, trembling aspen, sugar maple, laminated strand lumber (LSL) and laminated veneer lumber (LVL) were compared to 100% spruce-pine-fir group species (SPF) lumber made CLT panel. The bond line performance of the assemblies was tested as well as the dimensional stability and appearance of the panels when subjected to conditions with equilibrium moisture contents (EMC) of 4.5%, 12% and 16%. Results showed that higher density hardwood species were prone to delamination. LSL, LVL and trembling aspen yielded promising delamination results. Best overall dimensional stability results were achieved with EWP inclusive configurations. Aesthetic integrity assessment showed that the use of hardwood for the core layer and edge gluing of softwood outer layers had a negative impact. Overall, the study showed a great potential for manufacturing future composite CLT (CCLT) products using EWP and low density hardwood species. The cost premium of using these alternative materials would need to be offset by valuable sets of properties or by a reduction of the manufacturing cost.
A study was conducted with the primary objective of examining the efficacy of delamination test using cylindrical core specimens to assess the bond quality of cross laminated timber (CLT) products. A prototype coring drill bit was fabricated to prepare a cylindrical-shaped specimen, the height of which corresponds to the full thickness of the CLT panel. A secondary objective was to examine the effect of pressure, adhesive type, number of plies, and specimen shape on the delamination resistance of CLT panels. The wood material used for the CLT samples was Select grade nominal 1 x 6-inch Hem-Fir boards. Examples of three adhesive types were evaluated, which were designated as A, B, and C. The delamination tests used were as described in CAN / CSA O122-06 and EN 302-2.
Cylindrical specimen extracted as core was found satisfactory as a test specimen type for use in delamination testing of CLT product. Its efficacy was comparable to that of a square cross-section specimen. The former is recommended as it can be extracted from thicker panels and from any location in the panel. It would also be more convenient to plug the round hole.
Adhesive type had a strong effect on delamination resistance based on the two delamination tests used. Adhesive A exhibited the greatest delamination resistance, followed in decreasing order, by adhesives C and B. It should be noted that no effort was made to find the optimum CLT manufacturing parameters for each type of adhesive. Therefore the relative rankings of the adhesives tested may not be representative. However, for the purposes of this study, the different performance levels from the three adhesives are useful in providing insight into how the proposed delamination test responds to significant changes in CLT manufacturing parameters.
Pressure used in fabricating the CLT panel showed a strong effect on delamination resistance as demonstrated for one of the adhesives. Delamination resistance decreased with decreasing pressure. The effect of the number of plies in the CLT panel was dependent upon the type of adhesive, and this was probably related to the adhesive’s assembly time characteristic. These results provide support as to the effectiveness of delamination test in assessing the moisture durability of CLT panels. It was able to differentiate the performance in delamination resistance among different types of adhesives, and able to detect the effect of manufacturing parameters such as pressure and increased number of plies in CLT construction.
The test procedure described in CAN / CSA O122-06 appears to be reasonable in the delamination resistance assessment of CLT panels for qualification and quality control testing. Based on the results of the study along with some background information and guidelines, delamination requirements for CLT panels are proposed. The permitted delamination values are greater than those currently specified for laminated and fingerjoined lumber products. This is in recognition of the higher bond line stresses when bonded perpendicular laminations (i.e. CLT) are exposed to the delamination wetting and drying cycles, as opposed to parallel laminations (i.e. glulam or fingerjoints).
To explore the feasibility of hem-fir for CLT products, this work addressed the exploratory
and pilot plant studies of hem-fir cross-laminated timber (CLT) products through mechanical
tests. The hem-fir lumber was procured and then stress-graded based on dynamic modulus of
elasticity (MOE). The resulted 5-ply prototype CLT products were then tested non-destructively
and 3-ply pilot plant hem-fir CLT was tested destructively. The results showed that bending
performance of hem-fir CLT panel can be predicted. Considering cost-competitiveness and
end applications of hem-fir CLT products, the panel structure can be optimized based on the
stress-graded data of hem-fir lumber.
To better use the second-growth wood resources in value-added applications, this work addressed the manufacturing aspects of cross-laminated timber (CLT) products from western hemlock (Tsuga heterophylla (Raf.) Sarg) and amabilis fir (Abies amabilis (Dougl.) Forbes) (or hem-fir) harvested from coastal British Columbia, Canada. Small CLT billets (nominal 610 mm×610 mm) were made to examine CLT bond quality and durability through block shear and delamination tests. Two types of adhesives, single-component polyurethane (PUR) and emulsion polymer isocyanate (EPI) and two critical applied pressure parameters (0.28 and 0.83 MPa) were adopted to manufacture hem-fir CLT. It was found that the adhesive type and applied pressure significantly affected wood failure percentage (WFP) and delamination of hem-fir CLT. When PUR adhesive was used, CLT made at 0.83 MPa pressure yielded significantly higher WFP and lower delamination than that made at 0.28 MPa pressure. The results demonstrated that despite the fact that hem-fir lumber is not particularly specified in the current North American CLT standard, it could be used for manufacturing CLT with the required panel bond quality.
The objective of this study was to examine new attributes and conduct economic analyses for composite CLT (CCLT) and value-added appearance-based CLT products manufactured with varying substitution of softwood lumber with structural composite lumber (SCL) and hardwood lumber. Incentives for including such materials could be aesthetic, structural and economic.
Structural and aesthetic property assessments were carried out on prototype CLT panels. Multiple CLT panel configurations (17) were evaluated to assess the effects of including hardwood and SCL materials in the layups. Presence of hardwood in the panels’ configuration generally led to higher checking and density. Because of the higher shrinkage of hardwood, the bondline suffered from more delamination. A lower density hardwood (aspen) was included in some configurations and exhibited a greater direct compatibility with current Canadian manufacturing process. Changes to this process, such as selecting a hardwood specific adhesive may lead to improvements.
For the past decade, mountain pine beetle infestation in British Columbia, Canada, has substantially changed wood characteristics of vast amounts of the lodgepole pine (Pinus contorta) resource. Resin impregnation is one method that could improve the properties of the beetle-affected wood. The key objective of this study was to examine the impact of resin impregnation on dynamic MOE of lodgepole pine veneers and properties of laminated veneer lumber (LVL) made with these treated veneers. A new phenol formaldehyde resin was formulated to treat these veneers using dipping and vacuum-pressure methods. Five-ply LVL billets were made with treated and untreated veneers. Their color, dimensional stability, surface hardness, flatwise bending modulus and strength, and shear strength were evaluated. Good correlation existed between veneer MOE enhancement and resin solids uptake. With the same treatment, stained veneers had higher resin retention and in turn greater MOE enhancement than nonstained (clear) veneers. A 5-min dipping was sufficient for veneers to achieve approximately 7 and 10% resin solids uptake and in turn 5 and 8% enhancement in veneer MOE for nonstained and stained veneers, respectively. LVL made with treated veneers had a harder surface with no discoloration concerns compared with the control. Also, evidence suggested that use of resin impregnation can improve dimensional stability, shear strength, and flatwise bending MOE of LVL.